1. Field of the Invention
The invention relates to Orthogonal Frequency Division Multiplexing systems and more particularly to an efficient scheme for metric generation in OFDM receivers.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is well known as a highly spectral efficient transmission scheme capable of dealing with severe channel impairment encountered in a wireless environment. The basic idea of OFDM is to divide the available spectrum into several sub-channels (sub-carriers). By making all sub-channels narrowband, they experience almost flat fading, which makes equalization very simple. To obtain a high spectral efficiency the frequency response of the sub-channels are overlapping and orthogonal. This orthogonality can be completely maintained, even though the signal passes through a time-dispersive channel, by introducing a cyclic prefix (or guard interval). A cyclic prefix is a copy of the last part of the OFDM symbol which is pre-appended to the transmitted symbol. This makes the transmitted signal periodic, which plays a decisive roll in avoiding inter-symbol and inter-carrier interference.
OFDM signaling can largely eliminate the effects of inter-symbol interference for high-speed transmission in highly dispersive channels by separating a single high speed bit stream into a multiplicity of much lower speed bit streams each modulating a different sub-carrier. Fortunately, the apparently very complex processes of modulating (and demodulating) thousands of sub-carriers simultaneously are equivalent to Discrete Fourier Transform operations, for which efficient Fast Fourier Transform (FFT) algorithms exist, Thus integrated circuit implementations of OFDM demodulators are feasible for affordable mass-produced receivers. Furthermore, the use of error coding, interleaving, and channel-state information (CSI) allows OFDM signaling to function in a manner that is well suited to the needs of the terrestrial broadcasting channel. To combat frequency-selective fading and interference, channel decoding with soft-decision decoding can be properly integrated with an OFDM system. By means of interleaving the coded data before assigning them to OFDM sub-carriers at the modulator, clusters of errors caused by channel impairment can be broken up at the receiving end. The soft-decision decoding is carried out by a well known Viterbi decoder in an OFDM receiver. The Viterbi decoder is a sort of maximum likelihood decoder for the convolutional coding and must be fed with a soft decision comprising a measure or metric of the received signal. A metric can be made separately for each received bit to indicate a degree of confidence.
When data are modulated onto a single carrier in a time-invariant system, then a priori all data symbols suffer from the same noise power on average; the soft-decision information simply needs to take note of the random symbol-by-symbol variations that this noise causes. When data are modulated onto the multiple OFDM sub-carriers, the metrics become slightly more complicated as the various carriers will have different signal-to-noise ratios (SNR). For example, a carrier which falls into a notch in the frequency response will comprise mostly noise; one in the peak will suffer much less. Thus, in addition to the symbol-by-symbol variations, there is another factor to take account for in soft decisions: data conveyed by sub-carriers having a high SNR are a priori more reliable than those conveyed by sub-carriers having low SNR. This extra a priori information is usually known as channel-state information (CSI). The CSI concept can be extended to embrace interference which affects sub-carriers selectively. The inclusion of CSI in the generation of soft decisions is the key to the unique performance of OFDM in the presence of frequency-selective fading and interference.
OFDM has therefore been chosen for two recent standards for broadcasting—Digital Audio Broadcasting (DAB) and Digital Video Broadcasting for Terrestrial (DVB-T). Systems for DAB and DVB-T have been standardized by ESTI for use in Europe and elsewhere in the world. However, the existing mass-produced consumer products are not very cost-effective. It is shown that the system performance is heavily dependent on the OFDM receiver architecture. In particular, the most critical consideration is how to design and arrange the de-interleaving, metric generation as well as soft-decision decoding in an OFDM receiver. Accordingly, what is needed is a new and cost-effective architecture suitable for OFDM receivers.